[llvm] r348806 - debuginfo: Use symbol difference for CU length to simplify assembly reading/editing

[David Blaikie via llvm-commits]

Author: dblaikie
Date: Mon Dec 10 14:44:48 2018
New Revision: 348806

URL: http://llvm.org/viewvc/llvm-project?rev=348806&view=rev
Log:
debuginfo: Use symbol difference for CU length to simplify assembly reading/editing

Mucking about simplifying a test case ( https://reviews.llvm.org/D55261 ) I stumbled across something I've hit before - that LLVM's (GCC's does too, FWIW) assembly output includes a hardcode length for a DWARF unit in its header. Instead we could emit a label difference - making the assembly easier to read/edit (though potentially at a slight (I haven't tried to observe it) performance cost of delaying/sinking the length computation into the MC layer).

Reviewers: JDevlieghere, probinson, ABataev

Differential Revision: https://reviews.llvm.org/D55281

Modified:
    llvm/trunk/lib/CodeGen/AsmPrinter/DwarfFile.cpp
    llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
    llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.h
    llvm/trunk/test/DebugInfo/X86/sections_as_references.ll

Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfFile.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfFile.cpp?rev=348806&r1=348805&r2=348806&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfFile.cpp (original)
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfFile.cpp Mon Dec 10 14:44:48 2018
@@ -46,6 +46,8 @@ void DwarfFile::emitUnit(DwarfUnit *TheU
   TheU->emitHeader(UseOffsets);
 
   Asm->emitDwarfDIE(Die);
+
+  Asm->OutStreamer->EmitLabel(TheU->getEndLabel());
 }
 
 // Compute the size and offset for each DIE.

Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp?rev=348806&r1=348805&r2=348806&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp (original)
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp Mon Dec 10 14:44:48 2018
@@ -38,6 +38,7 @@
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetLoweringObjectFile.h"
+#include "llvm/Target/TargetMachine.h"
 #include <cassert>
 #include <cstdint>
 #include <string>
@@ -1553,7 +1554,17 @@ DIE *DwarfUnit::getOrCreateStaticMemberD
 void DwarfUnit::emitCommonHeader(bool UseOffsets, dwarf::UnitType UT) {
   // Emit size of content not including length itself
   Asm->OutStreamer->AddComment("Length of Unit");
-  Asm->emitInt32(getHeaderSize() + getUnitDie().getSize());
+  StringRef Prefix = isDwoUnit() ? "debug_info_dwo_" : "debug_info_";
+  MCSymbol *BeginLabel = Asm->createTempSymbol(Prefix + "start");
+  EndLabel = Asm->createTempSymbol(Prefix + "end");
+
+  // Use a label difference for the convenience of legible/easily modified
+  // assembly - except on NVPTX where label differences aren't supported.
+  if (Asm->TM.getTargetTriple().isNVPTX())
+    Asm->emitInt32(getHeaderSize() + getUnitDie().getSize());
+  else
+    Asm->EmitLabelDifference(EndLabel, BeginLabel, 4);
+  Asm->OutStreamer->EmitLabel(BeginLabel);
 
   Asm->OutStreamer->AddComment("DWARF version number");
   unsigned Version = DD->getDwarfVersion();

Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.h?rev=348806&r1=348805&r2=348806&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.h (original)
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.h Mon Dec 10 14:44:48 2018
@@ -48,6 +48,7 @@ protected:
 
   /// Target of Dwarf emission.
   AsmPrinter *Asm;
+  MCSymbol *EndLabel;
 
   // Holders for some common dwarf information.
   DwarfDebug *DD;
@@ -82,6 +83,7 @@ protected:
 public:
   // Accessors.
   AsmPrinter* getAsmPrinter() const { return Asm; }
+  MCSymbol *getEndLabel() const { return EndLabel; }
   uint16_t getLanguage() const { return CUNode->getSourceLanguage(); }
   const DICompileUnit *getCUNode() const { return CUNode; }
 

Modified: llvm/trunk/test/DebugInfo/X86/sections_as_references.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/DebugInfo/X86/sections_as_references.ll?rev=348806&r1=348805&r2=348806&view=diff
==============================================================================
--- llvm/trunk/test/DebugInfo/X86/sections_as_references.ll (original)
+++ llvm/trunk/test/DebugInfo/X86/sections_as_references.ll Mon Dec 10 14:44:48 2018
@@ -9,13 +9,19 @@
 ; CHECK-NOT: .L
 
 ; CHECK:      .section .debug_info
-; CHECK-NOT:  .L
-; CHECK:      .short 2             # DWARF version number
+; CHECK-NEXT: .long .Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
+; CHECK-NEXT: .Ldebug_info_start0:
+; CHECK-NEXT: .short 2             # DWARF version number
 ; CHECK-NOT:  .L
 ; CHECK:      .long .debug_abbrev  # Offset Into Abbrev. Section
 ; CHECK-NOT:  .L
 ; CHECK:      .long .debug_line    # DW_AT_stmt_list
 ; CHECK-NOT:  .L
+; CHECK:      .Ldebug_info_end0:
+; CHECK-NOT:  .L
+; CHECK:      .long .Ldebug_info_end1-.Ldebug_info_start1 # Length of Unit
+; CHECK-NEXT: .Ldebug_info_start1:
+; CHECK-NOT:  .L
 ; CHECK:      .long .debug_abbrev  # Offset Into Abbrev. Section
 ; CHECK-NOT:  .L
 ; CHECK:      .long .debug_line    # DW_AT_stmt_list
@@ -23,6 +29,7 @@
 ; CHECK:      .quad .debug_info+{{[0-9]+}} # DW_AT_type
 ; CHECK-NOT:  .L
 ; CHECK:      .byte 0              # End Of Children Mark
+; CHECK-NEXT: .Ldebug_info_end1:
 ; CHECK-NOT:  .L
 
 source_filename = "test/DebugInfo/X86/sections_as_references.ll"